The theme of the research supported by this grant has been, and continues to be improvement of the diagnostic and prognostic utility of the body surface electrocardiogram. This goal is reached with studies of ventricular recovery properties in normal and disease states and with development of methods of analysis to relate these properties to ECG T waveform and vulnerability to arrhythmias. Three groups of studies are described in this application. 1) Studies are to be done in which the dependence of ventricular recovery properties on activation sequence will be defined in normal and ischemic myocardium. Dependence of recovery on activation sequence is probably due to electrotonic interactions and is probably the reason the QRST area, which theoretically should be independent of activation if recovery properties remained the same, varies in clinical ECGs. Differences in behavior of the QRST with changes in activation are anticipated and may be an index of severity of ischemic heart disease. 2) Studies of the cycle length dependent changes in recovery properties in ischemic and non-ischemic tissues will be done. Behavior of recovery properties in response to single premature complexes in ischemic and normal myocardium has not been defined. Difference in behavior in the two kinds of tissue may be the mechanism for initiating tachyarrhythmias following PVCs with long coupling intervals. 3) A large percent of the events of ventricular recovery have no expression in the T wave. This cancellation of ECG effects is probably the mechanism for QT interval prolongation seen in canine ECGs during left stellate stimulation despite shortening ventricular recovery properties. Understanding of the relation of recovery properties to waveform during sympathetic manipulation is important in evaluation of patients with the prolonged QT interval. It is also important in evaluating exercise tests since the QT frequently prolongs although acute ischemia shortens recovery time. The studies outlined continue the themes of research supported by this grant and should improve the clinical utility of the ECG.